Many second and third generation wireless systems based on the Code Division Multiple Access (CDMA) technique employ fast power control on the uplink (e.g., from a mobile terminal to a base station) as well as the downlink (e.g., from a base station to a mobile terminal). Fast power control typically requires the receiver (either in the mobile terminal or the base station) to measure the local signal-to-noise ratio (SNR) during every transmission slot, compare it with a target SNR, and send a Transmit Power Control (TPC) command comprising one, or many, bits to the transmitter (also referred to herein as feedback bits). Upon receiving this TPC command, the transmitter then raises or lowers the transmit power by a fixed amount (in the dB domain). The transmission slots are, typically, of the order of a millisecond (e.g. 0.666 ms in UMTS (universal mobile telecommunications system), and 1.25 ms in IS-95 systems) so that the transmitter receives the feedback bits at a rate of about 1000 per second, allowing it to make rapid adjustments to the transmit power to counteract fading.
Unfortunately, during soft handoffs, fast power control gives rise to a transmit power drift problem in performing power control on the downlink. (Soft handoff is a condition in which a mobile terminal is in simultaneous communication with multiple base stations (or multiple sectors belonging to the same or different base stations in a sectorized setup). In a soft handoff, downlink transmissions to a mobile terminal are carried simultaneously by all of the base stations (or sectors) involved in the soft handoff whereas uplink signals (such as the above-mentioned TPC command) from the mobile terminal are independently received and decoded by these base stations (sectors).)
For example, let a mobile terminal be in soft handoff with K base stations: B1, B2, . . . BK, each having a respective common pilot channel transmit power: P1, P2, . . . , PK (in units of dBm (decibels relative to one milliwatt)). Further, let the downlink transmit powers on the user channels allocated by each of these base stations to communicate with the mobile terminal be: U1(t), U2(t), . . . , UK(t), respectively. (These powers are shown as functions of time, t, to emphasize the fact that they vary with time because of the above-mentioned power control.) Ideally, these downlink user channel transmit powers should vary in perfect synchrony. That is, if at time t the downlink transmit power on the first user channel associated with the first base station is equal to: U1(t)=P1−D, then, the downlink transmit powers on each of the other user channels should also be equal to: U2(t)=P2−D, U3(t)=P3−D, . . . , and UK(t)=PK−D, respectively. (Typically, the downlink transmit power U(t) for a user channel is measured relative to the (fixed) common pilot channel power, P, (or the maximum transmit power) for an associated base station (or sector) which is usually set to achieve certain coverage—hence, U(t)=P−D, where D is a measure of the relative amount.)
Such synchronous behavior is possible if all of the base stations (sectors) involved in the soft handoff started off with the same transmit power (measured relative to their respective common pilot channel powers), and then received the same TPC command from the mobile terminal. Unfortunately, this is not possible. Even though the TPC command is effectively broadcast by the mobile terminal to all of the base stations involved in soft handoff—the signal conveying the TPC command is affected in different ways by the time it reaches the receiver of each base station (sector). For instance, some base stations (sectors) may receive the resultant TPC command practically error-free (if the corresponding uplinks are good) whereas the TPC command received by some other base stations may be significantly corrupted by noise if the corresponding uplinks are weak. Also, any errors in the TPC command received by different base stations (sectors) are likely to be independent. As a consequence, even if all base stations (sectors) started with the same transmit powers (relative to their respective common pilot channel powers)—at some point in time the user channel transmit powers associated with each base stations may drift away from one another over time—hence, the transmit power drift problem.
To rectify the power drift problem, it has been suggested by those in the art that the base stations (sectors) involved in soft handoff with a mobile terminal periodically report their user channel transmit powers (measured relative to their respective common pilot channel powers) to a common control point such as Radio Network Controller (RNC) in a UMTS system. The common control point then decides on a reference power using the individual transmit powers provided by the base stations (sectors) involved in the soft handoff and sends a message carrying this reference power to all of these base stations (sectors). When the base stations (sectors) receive this message, they reset their user channel transmit powers to the reference power indicated in that message and resume normal operation including responding to fast power control feedback bits of the TPC command.